1. Field of the Invention
The present invention is directed generally at filtering wavelengths, and more specifically at filtering multiplexed wavelengths using an electrically tunable Fabry-Perot filter.
2. Information Disclosure Statement
A key component of Wavelength Division Multiplexed (WDM) networks is a tunable broadband wavelength demultiplexer. The ideal wavelength filter would be widely tunable with constant transmittance over the entire free spectral range, would be quickly tunable with narrow channel spectrum selectivity (high finesse), and would be integrable with other lightwave components such as amplifiers or detectors. To that end, many different devices have been developed as demultiplexers, such as a piezoelectrically tunable fiber Fabry-Perot filter and a laser diode operated below threshold.
A piezoelectrically tunable Fabry-Perot fiber is described in J. Stone and L. W. Stulz, "Pigtailed High-Finesse Tunable Fibre Fabry-Perot Interferometers With Large, Medium and Small Free Spectral Ranges," Elect. Lett., Vol. 23, pp. 781-783 (1987). This article describes three Fabry-Perot filter prototypes and their test results. All three designs used standard PZT piezoelectric components for tuning. These filters yielded finesse values up to 200. Furthermore, an insertion loss as low as 1.5 dB was observed for lower finesse values. Thus, mechanically adjusted filters have excellent wavelength selectivity and low insertion loss. Since they are mechanically tuned, however, they can only attain tuning speeds in the order of milliseconds. Therefore, a need exists for faster tuning filters.
Semiconductor devices with inherently faster tuning speed have been reported by a number of groups For example, T. Numai et al., "1.5 .mu.m Tunable Wavelength Filter with Wide Tuning Range and High Constant Gain Using a Phase-controlled Distributed Feedback Laser Diode," Appl. Phys. Lett., Vol. 53, No. 13 (Sep. 26, 1988), and Katsuaki Magari, "Optical Signal Selection with a Constant Gain and a Gain Bandwidth by a Multielectrode Distributed Feedback Laser Amplifier" Appl. Phys., Lett., Vol. 51, No. 24 (Dec. 14, 1987) describe laser diodes with wide tuning ranges and high tuning speed. However, laser filters suffer from gain instability. Gain instability arises in laser diodes because they absorb light at the wavelengths typically used in multiplexing, e.g., 1.55 .mu.m.
To overcome this problem and achieve a "transparent" condition, current is applied across the laser diode. This excites the electrons so that they cannot absorb the light energy. Although this creates a transparent condition, the diode becomes highly excited. In fact, the diode is biased just below the threshold current of the laser.
Because the bias hovers just below the threshold current, a slight energy change may cause the diode to cross the threshold and become a laser emitter. The transmission of light, for example, may provide the requisite energy for the diode to cross the laser threshold. Additionally, because tuning requires current injection, the tuning process may push the diode over the laser threshold. If the diode crosses the threshold and becomes an emitter, significant damage may result.
In addition to instability, a laser diode suffers from tuning shortcomings. For example, the shape of the transmission spectrum varies as the current varies. Furthermore, current variations produce non-linear shifts in the transmission light wavelengths. Both of these shortcomings hinder the tunability of a laser diode filter.
Thus, the need remains for a wavelength filter with not only quick tunability, but also gain stability, constant filter shape, and linear tuning. The present invention fulfills this need.